As illustrated in FIG. 1, in a general twin roll strip caster 100, molten steel in a tundish 103 is supplied between two casting rolls 101 and an edge dam 102 attached to opposite surfaces thereof via an immersion nozzle 104, to form molten steel pool 105, and the casting rolls 101 are rotated in opposite directions relative to each other such that the molten steel is brought into contact with the rolls to thus transfer heat into the rolls, thereby rapidly solidifying the molten steel, by which a cast product 106 is manufactured.
In the strip caster 100 for manufacturing the cast product 106, the shape of the surface of the roll is regarded as very important, and the related technique is the core of the strip casting process for manufacturing a cast product having no defects. Generally, in the case where liquid metal is solidified to a solid phase, internal stress, including solidification contraction force and transformation stress, is generated. In particular, in the case where solidification occurs at a high cooling rate, as in the strip casting process, there is a very high probability of surface cracks being created in the cast product, attributable to the increase in such internal stress. Thus, in the strip casting process, a technique for imparting the surface of the casting roll with roughness to effectively disperse and eliminate the internal stress is typically known.
When subjecting general steel to strip casting along with austenitic stainless steel, which is the most common type of steel for the strip casting process to date, examples of methods of treating the surface of the casting roll include knurling, photo-etching, shot blasting, and laser processing. The roughened surface, resulting from the above process, is featured in that it is formed in the shape of dimples 210, which are fine and independent, as seen in FIG. 2. This shape functions to effectively disperse and eliminate stress accompanied by solidification to thereby prevent the generation of surface cracks in the cast product.
However, in the case where a strip of high Mn steel is manufactured using the casting roll which is imparted with roughness through the above process, small dents 310, which are not observed in other types of steel, are generated in the surface of the cast product, as seen in FIG. 3(a).
The reason why the dents 310 are generated is that, in high Mn steel, unlike the other types of steel, Mn, which has a lower melting point and lower vapor pressure to thus be more easily volatilized than the other elements, is contained in an amount of 18˜25 wt % in molten steel, and is therefore easily evaporated from the molten steel pool during the casting, such that the resultant vapor gas is fully loaded in a meniscus shield. Further, the vapor gas is deposited on the surface of the casting roll, as well as the inner surface of the meniscus shield. When the vapor gas, which is in a state of being collected in the isolated dimple of the surface of the rotating casting roll, is brought into contact with the molten steel in a liquid phase, expansion or explosion of the gas loaded in the dimple may occur due to high heat transferred from the molten steel. Accordingly, during solidification, an impact is applied to the weak solidification shell, which is in direct contact with the upper surface of such a dimple, undesirably generating the dent 310 by the force of impact. FIG. 3(b) is a graph illustrating the number of dents and the dent index when manufacturing a strip of high Mn steel using a conventional casting roll. As shown in this drawing, the case where a cast product of high Mn steel is manufactured using the conventional casting roll results in a large number of dents and a high dent index.
Therefore, with the goal of preventing the generation of undesirable surface dents, there is required a novel dimple forming method, which is quite different from conventional roll surface treatment for a process of casting austenitic stainless steel or general steel to a strip.